1. Field of the Invention
The current invention relates to methods for fabricating large area, low defect density semiconductor substrates and devices. Such substrates are the basis of optoelectronics and microelectronics device applications, such as light emitting diodes, laser diodes, and high electron mobility transistors.
2. Description of the Related Art
Metallic nitride, in particular Groups III-V nitride semiconductors have been widely used in UV and blue to green light emitting diodes and short wavelength laser diode applications. They are also a very important material in high electron mobility devices. The most popular growth method for GaN is by vapor-phase synthesis, for example metalorganic chemical vapor deposition (MOCVD), hydride vapor-phase epitaxy (HVPE), molecular-beam epitaxy (MBE), metal-organic chloride (MOC), and so on. Since large III-nitride substrates are not readily available, the III-nitride semiconductor devices have often been heteroepitaxially grown upon different substrates, for example sapphire, silicon carbide (SiC), silicon (Si), gallium arsenide (GaAs), and so on. Among them, sapphire is the most widely used substrate (template) due to its stable material properties.
The lattice constant mismatch between GaN and sapphire is however relatively large. This poses a big challenge to grow a low defect crystal material. In addition, the thermal expansion coefficient mismatch between GaN and substrates can also introduce stress into GaN thin film during growth process and result in micro cracks in the film when the substrate temperature is cycled in the growth process. These mismatched factors lead to a large number of defects, as large as 108-9 cm−2, in the grown GaN material and can significantly affect the performance of the devices subsequently fabricated on top of it. It is very important to reduce the number of crystal defects to improve the device performance. From application point of view, the epitaxial substrates that are used for growing GaN may also have some unwanted properties that can seriously limit the device applications and/or mass production, for example, low thermal conductivity, not electrically conducting, hard to cleave, etc. It is therefore desirable to remove the substrate from the grown GaN material as early as possible in the steps of fabrication processes.
There have been various methods to remove sapphire substrate (template) from the grown GaN thin film. These methods include mechanical grinding, chemical etching, interfacial decomposition, and interfacial structure fracturing. The mechanical grinding process is time consuming because sapphire is a fairly hard material and it requires precise handling to achieve large area uniformity. Chemical etching, either wet or dry etching, is a difficult and slow process because sapphire is a relatively chemical inert material. The interfacial decomposition that uses laser ablation to remove a thin layer of GaN at the sapphire interface is a sequential process, therefore, is also a time consuming process. This approach requires an expansive UV laser equipment. The interfacial fracturing approach requires extra micro fabrication processes to create a mechanically weak layer prior the GaN growth.
Therefore, there is a need for a simple, fast, easy, and/or low-cost process of making a low defect density and/or large area metallic nitride (in particular III-V nitride) semiconductor substrate or device.